Conventional cardiopulmonary bypass uses an extracorporeal blood circuit which includes a venous drainage line, venous reservoir, blood pump, oxygenator, and arterial filter. Blood circulation is accomplished by draining blood from the patient by gravity through the venous drainage line to the venous reservoir. From there, blood drains down to the blood pump, placing this portion of the circuit at a negative pressure with respect to atmosphere. The pump supplies positive pressure to return the blood to the patient through the oxygenator and filter. The venous reservoir holds blood volume as required, while both the venous reservoir and arterial filter remove air bubbles from the blood. These may cause health problems if returned to the patient in the arterial blood flow. Air can enter the circuit from a number of sources, including around the venous cannula and through various unanticipated intra-operative events. A further complication arises if a centrifugal pump is used, in which case a large volume of air will de-prime the pump, depriving it of its pumping capability.
In order to remove air from an extracorporeal circuit prior to its use, the circuit is primed with an appropriate solution. During surgery, this solution dilutes the patient's blood, and it is therefore desirable to minimize the volume required. The venous reservoir contains a relatively large volume of fluid, and recently it has been proposed to eliminate this component of the circuit. Several problems arise, however. Without the venous reservoir between the patient and the oxygenator, any air in the venous line will either accumulate in the centrifugal pump (if used) or be pumped into the oxygenator. Furthermore, if a large bolus of air is introduced, it may de-prime the pump and oxygenator. Although arterial filters are designed to capture air bubbles, they are not designed to handle larger volumes of air such as may occur from the causes described above. Also, arterial filters are located downstream of both the pump and the oxygenator, and therefore cannot prevent air problems that would occur in those devices. Furthermore, conventional arterial filters are designed to operate at positive blood pressures.